Chapter 24 – Black Holes and Spacetime

Question-and-answer flashcards covering key figures, principles of general relativity, black-hole properties, and gravitational-wave discoveries from the Chapter 24 lecture.

What kind of object is Cygnus X-1 and why does it emit strong X-rays?

Cygnus X-1 is a stellar-mass black hole in a binary system; gas pulled from its massive blue companion forms a hot accretion disk that glows in X-rays before falling in or being ejected in jets.

According to the equivalence principle, what do you experience while in free fall?

You feel weightless, because inside a freely falling frame gravity is locally indistinguishable from the absence of gravity.

How does your apparent weight change in an elevator that accelerates downward versus upward?

You feel lighter when the elevator accelerates downward and heavier when it accelerates upward.

Why do astronauts inside an orbiting Space Shuttle appear to float?

The shuttle and everything in it are in continuous free fall around Earth, producing a state of weightlessness relative to the spacecraft.

How does general relativity say gravity affects a beam of light?

Gravity curves spacetime, so a light beam follows a curved path in a gravitational field rather than a straight Euclidean line.

In a spacetime diagram, what do the horizontal and vertical axes represent?

Horizontal represents distance (space), and vertical represents elapsed time, showing how events unfold in spacetime.

Why does Mercury’s orbit precess more than predicted by Newtonian gravity?

The Sun’s strong spacetime curvature adds extra precession, precisely accounted for by Einstein’s general theory of relativity.

What is gravitational lensing?

The bending of light by a massive object, causing apparent shifts, magnification, or multiple images of background sources.

What is the Shapiro time delay observed for radio waves passing near the Sun?

Radio signals take slightly longer to reach us because spacetime is curved near the Sun, lengthening their path compared with flat space.

What is the event horizon of a black hole?

The spherical boundary from which neither matter nor light can escape because the escape velocity equals or exceeds the speed of light.

As a massive stellar core collapses, how does its escape velocity change?

Escape velocity increases; once it exceeds the speed of light the object becomes a black hole.

Which physicist popularized the term “black hole” in the late 1960s?

John Wheeler.

What is the Schwarzschild radius?

The radius of the event horizon for a non-rotating black hole, first calculated mathematically by Karl Schwarzschild.

How do accretion disks and jets form in a black-hole binary system?

Gas from the companion star spirals in, forming a hot, rotating disk; magnetic and centrifugal forces can channel some material into narrow jets perpendicular to the disk.

What does LIGO detect, and how long are its arms?

LIGO measures gravitational waves using two perpendicular 4-kilometer laser interferometer arms at each site.

What did the first LIGO detection confirm about Einstein’s theory?

The observed waveform from a black-hole merger matched general-relativity predictions, confirming gravitational waves and the theory’s accuracy.

Why can a beam of light emitted just above a near-black-hole surface curve back down?

Strong spacetime curvature bends most light paths back toward the mass; only a perfectly upward beam could escape when the surface is just outside the event horizon.

How did the 1919 solar-eclipse observations support general relativity?

They measured deflection of starlight passing near the Sun, matching Einstein’s predicted bending by spacetime curvature.

What do gravitational-wave observations of black-hole mergers reveal about black-hole masses?

They provide direct measurements of the individual black holes’ masses and the larger mass of the merged remnant.

Why have some LIGO-detected black holes surprised astronomers?

Their unexpectedly large masses suggest formation channels or physics not fully explained by existing stellar-evolution models, hinting that new ideas may be needed.